Curing agents, or hardeners, are employed to react with a resin monomer, such as epoxy, isocyanate, acid anhydride etc., to produce a cured polymeric resin.
The resulting cured resins are employed in a wide range of industries and in a wide range of applications. The chemical and physical properties of the resulting cured resins vary widely, primarily depending upon the choice of monomer and of the curing agent.
There is ongoing demand for curable resin systems which can provide improved physical and chemical properties, particularly for use in demanding applications, such as for use in aerospace composite materials.
In one common application, a liquid blend of resin monomer and curing agent is injected or infused into a fibrous reinforcing structure, e.g. in the so-called resin transfer moulding or infusion processes. This involves preparing a liquid blend comprising both the curable resin and curing agent at an elevated temperature so as to reduce the viscosity ready for infusion. The curing agent must therefore have low reactivity to prevent premature reaction occurring before infusion takes place. Following infusion, the composite material produced is cured by exposure to elevated temperature to produce the cured composite material.
Traditionally, such liquid blends are produced as a one-component system, combining the resin monomer and curing agent intimately mixed together. This is convenient as it allows the end user merely to introduce one composition into the fibrous reinforcement. Such one-component systems are typically semi-solids at room temperature and only become liquid at an elevated temperature, e.g. from 60° C. to 100° C., when required for use. As the curing agent and the resin are together in the same material, a curing agent must be selected to have low reactivity. The low reactivity must also be maintained at the increased temperature when the composition becomes a flowable liquid.
A particularly convenient type of curing agent are the aromatic amines, as these provide good structural performance together with low reactivity. The known aromatic amines which give good mechanical performance in the cured resin are all solid at room temperature.
However, the inherent reactivity between resin monomer and curing agent can never be removed entirely, even when in solid form, and this limits the volume of material that can be transported and presents thermal hazard situations. Additionally, if a UN4.1 transportation category is defined, then, for such one-component curable resin compositions, sea transportation is difficult and air transportation forbidden.
Further improvements in the area of curable resin compositions which can produce cured resins suitable for use in structural applications would therefore be highly desirable.